The subject matters disclosed in this specification are related to the subject matters disclosed in the following copending, commonly-assigned U.S. patent applications:
(1) U.S. Ser. No. 09/136,739 filed by Shinji Imai on Aug. 19, 1998, now U.S. Pat. No. 6,268,614 and entitled xe2x80x9cELECTROSTATIC RECORDING MEMBER, ELECTROSTATIC LATENT IMAGE RECORDING APPARATUS, AND ELECTROSTATIC LATENT IMAGE READ-OUT APPARATUS,xe2x80x9d corresponding to Japanese patent application No. 10-232824, which is disclosed in Japanese Unexamined Patent Publication No. 2000-105297; and
(2) U.S. Ser. No. 09/385,443 filed by Satoshi Arakawa on Aug. 30, 1999 and entitled xe2x80x9cRADIATION IMAGE DETECTING SYSTEM,xe2x80x9d corresponding to Japanese patent application No. 10-243379, which is disclosed in Japanese Unexamined Patent Publication No. 2000-137080.
The contents of the above copending, commonly-assigned U.S. patent applications and the corresponding Japanese patent applications are incorporated in this specification by reference.
1. Field of the Invention
The present invention relates to an energetic-beam detection apparatus which absorbs an energetic beam by using an energetic-beam absorber made of selenium, where the energetic beam may be a beam of light, X rays, gamma rays, any other electromagnetic waves having shorter or longer wavelengths, and energetic particles.
2. Description of the Related Art
In various systems which have been proposed or used, an energetic-beam detection apparatus including a selenium detector is used, where the selenium detector is made of selenium as an energetic-beam absorber which is sensitive to energetic beams. In the field of medical radiography, radiographic image readout systems using an energetic-beam detection apparatus which can efficiently detect radiation have been proposed in order to decrease radiation doses to which patients are exposed, and improve performance in diagnosis. In the above radiographic image readout systems, charges having an amount corresponding to the intensity of radiation which has passed through a subject (patient) is stored as latent-image charges in a photoconductive layer in a solid-state radiation detector so that a radiographic image is recorded, where the solid-state radiation detector is a kind of selenium detector. There are two methods of reading out an image signal which represents the amount of the latent-image charges, the TFT readout method and the optical readout method.
Since the above photoconductive layer exhibits conductivity when the photoconductive layer is exposed to radiation such as X rays, the photoconductive layer is also called an X-ray photoconductive layer. However, in this specification, the term xe2x80x9cphotoconductive layerxe2x80x9d is used in its broadest sense, i.e., the term xe2x80x9cphotoconductive layerxe2x80x9d covers any photoconductive layers which exhibit conductivity when the photoconductive layers are exposed to light, X rays, gamma rays, or any other electromagnetic radiation having a shorter or longer wavelength.
According to the TFT readout method, TFTs (thin-film transistors) are scanned and activated, the latent-image charges stored in the photoconductive layer is converted into a radiographic image signal, which is then output. For example, the coassigned U.S. patent application Ser. No. 09/385,443 corresponding to Japanese Unexamined Patent Publication No. 2000-137080 discloses a solid-state radiation detector which is constructed by forming a first electrode, a photoconductive layer, a plurality of charge collecting electrodes, a capacitor array, a TFT array, and a second electrode in this order on a fluorescent layer. In the solid-state radiation detector, the fluorescent layer emits visible light when the fluorescent layer is exposed to radiation. The first electrode is transparent to the radiation and the visible light. The photoconductive layer contains a-Se (amorphous selenium) as a main component, and has a thickness of about 400 micrometers. The plurality of charge collecting electrodes respectively correspond to pixels, and are arranged in the form of a matrix with a predetermined pitch on an insulator substrate being made of quartz glass and having a thickness of 3 mm. The capacitor array includes a plurality of capacitors each of which stores as latent-image charges signal charges collected by a corresponding one of the plurality of charge collecting electrodes. The TFT array includes a plurality of TFTs, each of which transfers the latent-image charges stored in a corresponding one of the plurality of capacitors to a detection circuit. For example, the fluorescent layer contains Gd2O2S:Tb as a main component, and has a thickness of about 100 micrometers. It is preferable to arrange the fluorescent layer in contact with or in the vicinity of the first electrode. The photoconductive layer generates charges when the photoconductive layer is exposed to the above visible light as well as the above radiation which carries image information.
When the fluorescent layer is exposed to the radiation which carries image information, a portion of the radiation is converted into visible light in the fluorescent layer. The remaining portion of the radiation and the visible light converted from the radiation enter the photoconductive layer through the first electrode. Since the photoconductive layer generates charges when the photoconductive layer is exposed to either of visible light and radiation, charges corresponding to the image information carried by the visible light and the remaining portion of the radiation are generated in the photoconductive layer when the visible light and the remaining portion of the radiation enter the photoconductive layer. Then, the generated charges are read out through the TFTs. The above solid-state radiation detector is advantageous in that a high-quality radiographic image can be obtained. Since the photoconductive layer in the above solid-state radiation detector contains a-Se as a main component, the solid-state radiation detector can be regarded as selenium detector.
On the other hand, according to the optical readout method, the latent-image charges stored in the photoconductive layer are converted into an image signal by applying reading light to the solid-state radiation detector, and then the image signal is read out. For example, the optical readout method is disclosed in U.S. Pat. Nos. 4,176,275, 5,268,569, 5,354,982, 4,535,468, and 4,961,209, Research Disclosure No. 23027, June 1983 (xe2x80x9cMethod and device for recording and transducing an electromagnetic energy patternxe2x80x9d), Japanese Unexamined Patent Publication No. 9(1997)-5906, and Medical Physics, Vol. 22, No. 12 (xe2x80x9cX-ray imaging using amorphous seleniumxe2x80x9d).
For example, U.S. Pat. No. 4,535,468 discloses a solid-state radiation detector which is constructed by forming a recording-side photoconductive layer, an intermediate layer (trap later), a reading-side photoconductive layer, and a reading-side electrode layer in this order on a recording-side electrode layer. The recording-side electrode layer is relatively thick (2 mm thick) and made of aluminum, and behaves as a conductive substrate which is transparent to a recording electromagnetic radiation (hereinafter called recording light). The recording-side photoconductive layer contains a-Se (amorphous selenium) as a main component, and has a thickness of 100 to 500 micrometers. The intermediate layer (trap later) is made of AsS4, As2S3, As2Se3, or the like, and has a thickness of 0.01 to 10.0 micrometers. Latent-image charges generated in the recording-side photoconductive layer are trapped and stored in the intermediate layer (trap later). The reading-side photoconductive layer contains a-Se (amorphous selenium) as a main component, and has a thickness of 0.5 to 100 micrometers. The reading-side electrode layer is made of gold or ITO (indium tin oxide), has a thickness of 100 nm, and behaves as a conductive substrate which is transparent to a reading electromagnetic radiation (hereinafter called reading light). The above solid-state radiation detector is advantageous in that the dark resistance and the response speed in reading are high. Since the recording-side and reading-side photoconductive layers in the above solid-state radiation detector contain a-Se as a main component, the solid-state radiation detector can also be regarded as selenium detector.
In order to increase the S/N ratio, and decrease the readout time by parallel reading from pixels arranged in the main scanning direction, the reading-side electrode layers in some solid-state radiation detectors include a striped electrode array comprised of a number of elements (linear electrodes) arranged at a pixel pitch, for example, as disclosed in the coassigned U.S. patent application Ser. No. 09/136,739, U.S. Pat. No. 6,268,614 and Japanese Unexamined Patent Publication No. 2000-105297. However, when layers which constitute the solid-state radiation detector disclosed in U.S. Pat. No. 4,535,468 are formed, the recording-side electrode layer must be formed in the final stage of the manufacturing process, i.e., after the reading-side photoconductive layer is formed. It is difficult to form the striped electrode array in the above solid-state radiation detector for the following reason. When the striped electrode array is formed by lithography, a photoetching process, which is used in the manufacture of the semiconductor, is necessary. However, the photoetching process usually includes a high-temperature process such as the process of baking photoresist (e.g., at the temperature of 200xc2x0 C.), and the a-Se which constitutes the photoconductive layers cannot withstand such a high temperature. Therefore, the characteristics of the photoconductive layers deteriorate during the photoetching process.
In addition, since the alkali developer solution used in the process of developing the photoresist generates harmful gas on contact with the a-Se, special provision for elimination of the harmful gas increases the complexity of the manufacturing process and the manufacturing cost.
The coassigned U.S. patent application Ser. No. 09/136,739, U.S. Pat. No. 6,268,614 corresponding to Japanese Unexamined Patent Publication No. 2000-105297 proposes a solid-state radiation detector which is constructed by forming a recording-side electrode layer, a recording-side photoconductive layer, a charge transport layer, a reading-side photoconductive layer, and a reading-side electrode layer in this order. The recording-side electrode layer includes a SnO2 film (i.e., the so-called NESA film) which is transparent to radiation as recording light. The recording-side photoconductive layer contains a-Se as a main component. The charge transport layer is made of, for example, a-Se doped with an organic substance or chlorine of 10 to 200 ppm, and forms a charge storage region at the interface with the recording-side photoconductive layer so that latent-image charges generated in the recording-side photoconductive layer are stored in the charge storage region. The reading-side photoconductive layer contains a-Se as a main component. The reading-side electrode layer is transparent to reading light.
Generally, the layers which constitute the above solid-state radiation detector can be formed in either the above-mentioned order or the reverse order. However, specifically, the coassigned U.S. patent application Ser. No. 09/136,739, U.S. Pat. No. 6,268,614 corresponding to Japanese Unexamined Patent Publication No. 2000-105297 proposes that the reading-side electrode layer includes a striped electrode array (or a comb electrode) formed on a transparent glass substrate (as a support) with a conductive substance such as the NESA film, and is used as a positive electrode, where the striped electrode array (or comb electrode) is comprised of a number of stripe electrodes (or teeth electrodes) arranged at a very small pixel pitch. That is, the number of stripe electrodes (or teeth electrodes) must be formed at a sufficiently small pitch by using the semiconductor processing technique. Therefore, in this case, the above striped electrode array or comb electrode is required to be firstly formed on the transparent glass substrate by photoetching or the like, and thereafter the other layers are formed over the striped electrode array (or comb electrode). In addition, in order to achieve high sharpness and S/N ratio in medical radiography, the pixel pitch of 50 to 200 micrometers is required.
On the other hand, the inventor of the present application found that a very fine stripe pattern can be produced at a low cost when a relatively thin (50 to 200 nm thick) ITO film is formed on the transparent glass substrate before forming the above striped electrode array in the reading-side electrode layer.
As described above, the solid-state radiation detector disclosed in the coassigned U.S. patent application Ser. No. 09/136,739, U.S. Pat. No. 6,268,614 and Japanese Unexamined Patent Publication No. 2000-105297 is advantageous in that the dark resistance and the response speed in reading are high. In addition, since two layers which contain a-Se as a main component, i.e., the recording-side and reading-side photoconductive layers, are used energetic-beam absorbers, this solid-state radiation detector can also be regarded as a selenium detector. In addition, the glass substrate is, for example, a Corning 1737 glass plate having a thickness of 1.1 mm. In this case, the effective medium size is 20xc3x9720 cm or greater. In particular, the effective medium size for breast X-ray imaging is 43xc3x9743 cm.
However, when the above-mentioned selenium detectors (solid-state radiation detectors) are used for a long time in a high-temperature environment, the temperature of the solid-state radiation detectors increases. On the other hand, in cold climates, the selenium detectors may be placed in a low-temperature environment, for example, during storage.
Generally, the dark resistance characteristics of amorphous selenium films tend to deteriorate over time even under normal use. This is because the so-called bulk crystallization develops over time. Therefore, the practical lifetimes of the selenium detectors are limited. In particular, the bulk crystallization is accelerated when the temperature exceeds 45xc2x0 C. Further, when the temperature exceeds 50xc2x0 C., the selenium films are softened, and the practical lifetimes can be seriously reduced.
Furthermore, the coassigned U.S. patent application Ser. No. 09/136,739, U.S. Pat. No. 6,268,614 and Japanese Unexamined Patent Publication No. 2000-105297 also disclose a selenium detector in which a selenium multilayer film as a photoconductive layer is formed on a glass substrate. In this selenium detector, great thermal stress is produced at the boundary between the glass substrate and the selenium multilayer film due to the difference (by about an order of magnitude) in the thermal expansion coefficient at low temperature between the glass substrate and the selenium multilayer film, and the selenium multilayer film is likely to be separated from the glass substrate by exfoliation. In particular, when the operation of the selenium detector is suddenly started in a situation in which thermal stress is caused at the boundary between the glass substrate and the selenium multilayer film by low temperature, and the selenium multilayer film is not yet separated from the glass substrate by exfoliation, exfoliation of the selenium multilayer film is likely to occur since a temperature difference between the selenium multilayer film and the glass substrate is caused by the start of the operation.
An object of the present invention is to provide a method for controlling temperature in an energetic-beam detection apparatus including a selenium detector comprised of a substrate and an energetic-beam absorber which is formed on the substrate and contains selenium, whereby the temperature is controlled so as to suppress decrease in the practical lifetime of a selenium detector due to softening or development of bulk crystallization of selenium in a high-temperature environment, and prevent exfoliation of the energetic-beam absorber from the substrate due to thermal stress caused by operation in a low-temperature environment.
Another object of the present invention is to provide an energetic-beam detection apparatus including a selenium detector comprised of a substrate and an energetic-beam absorber which is formed on the substrate and contains selenium, wherein decrease in the practical lifetime of a selenium detector due to softening or development of bulk crystallization of selenium in a high-temperature environment can be suppressed, and exfoliation of the energetic-beam absorber from the substrate due to thermal stress caused by operation in a low-temperature environment can be prevented.
(1) According to the first aspect of the present invention, there is provided a method for controlling temperature in an energetic-beam detection apparatus including a selenium detector which includes a substrate and an energetic-beam absorber being formed on the substrate and containing selenium. The method comprises the steps of: (a) detecting a temperature of the selenium detector; and (b) controlling the temperature of the selenium detector so that the temperature is maintained in a range of 0xc2x0 C. to 50xc2x0 C. when the selenium detector is in operation.
Preferably, the method according to the first aspect of the present invention also has one or any possible combination of the following additional features (i) and (ii).
(i) In the step (b), the temperature may be maintained in a range of 10xc2x0 C. to 45xc2x0 C. when the selenium detector is in operation.
(ii) In the step (b), the temperature may be maintained in a range of 30xc2x0 C. to 40xc2x0 C. when the selenium detector is in operation.
Since the temperature of the selenium detector can be maintained in the range of 0xc2x0 C. to 50xc2x0 C. when the selenium detector is in operation, it is possible to prevent serious decrease in the practical lifetime of the selenium detector due to softening or development of bulk crystallization of selenium in a high-temperature environment, and exfoliation of the energetic-beam absorber from the substrate due to thermal stress caused by operation in a low-temperature environment. Further, when the temperature of the selenium detector is maintained in the range of 30xc2x0 C. to 40xc2x0 C., the decrease in the practical lifetime of the selenium detector can be further effectively suppressed, and the exfoliation of the energetic-beam absorber from the substrate can be further effectively prevented. In addition, when the temperature of the selenium detector is maintained in the range of 30xc2x0 C. to 40xc2x0 C., the energetic-beam detection apparatus can be used in a temperature range in which the detection sensitivity is high. Therefore, the detection sensitivity of the energetic-beam detection apparatus is improved.
(2) According to the second aspect of the present invention, there is provided an energetic-beam detection apparatus comprising: a selenium detector which includes a substrate and an energetic-beam absorber being formed on the substrate and containing selenium; a temperature detecting unit which detects a temperature of the selenium detector; and a temperature control unit which controls the temperature of the selenium detector so that the temperature is maintained in a predetermined range when the selenium detector is in operation.
Due to the provision of the temperature control unit, the temperature of the selenium detector can be maintained in the predetermined range when the selenium detector is in operation. Therefore, the decrease in the practical lifetime of the selenium detector due to softening or development of bulk crystallization of selenium in a high-temperature environment can be automatically suppressed, and the exfoliation of the energetic-beam absorber from the substrate due to thermal stress caused by operation in a low-temperature environment can be automatically prevented. Thus, users can continue to use the energetic-beam detection apparatus without being bothered with the temperature control of the selenium detector. That is, the usability of the energetic-beam detection apparatus is improved.
Preferably, the energetic-beam detection apparatus according to the second aspect of the present invention also has one or any possible combination of the following additional features (iii) to (v).
(iii) The predetermined range may be from 0xc2x0 C. to 50xc2x0 C.
(iv) The predetermined range may be from 10xc2x0 C. to 45xc2x0 C. In this case, the decrease in the practical lifetime of the selenium detector can be further effectively suppressed, and the exfoliation of the energetic-beam absorber from the substrate can be further effectively prevented.
(v) The energetic-beam absorber may be realized by a selenium multilayer film having a thickness of 150 to 1,500 micrometers, where the selenium multilayer film includes a plurality of selenium films. In this case, high detection sensitivity can be achieved in a wide energy range of energetic beams.
(3) According to the third aspect of the present invention, there is provided an energetic-beam detection apparatus comprising: a selenium detector which includes a substrate and an energetic-beam absorber being formed on the substrate and containing selenium; a temperature detecting unit which detects a temperature of the selenium detector; and an operation suppressing unit which suppresses the operation of the selenium detector when the temperature of the selenium detector is outside a predetermined range.
Since the operation suppressing unit suppresses the operation of the selenium detector when the temperature of the selenium detector is outside a predetermined range, the selenium detector can operate only when the temperature of the selenium detector is within the predetermined range. Thus, the decrease in the practical lifetime of the selenium detector due to softening or development of bulk crystallization of selenium in a high-temperature environment can be suppressed, and the exfoliation of the energetic-beam absorber from the substrate due to thermal stress caused by operation in a low-temperature environment can be prevented.
Preferably, the energetic-beam detection apparatus according to the third aspect of the present invention also has one or any possible combination of the aforementioned additional features (iii) to (v).
(4) According to the fourth aspect of the present invention, there is provided an energetic-beam detection apparatus comprising: a selenium detector which includes a substrate and an energetic-beam absorber being formed on the substrate and containing selenium; a temperature detecting unit which detects a temperature of the selenium detector; and a notification unit which notifies a user of deviation of the temperature of the selenium detector from a predetermined range when the temperature of the selenium detector is outside the predetermined range.
Since the notification unit notifies a user of deviation of the temperature of the selenium detector from a predetermined range when the temperature of the selenium detector is outside the predetermined range, the user can take appropriate measures to suppress the decrease in the practical lifetime of the selenium detector due to softening or development of bulk crystallization of selenium in a high-temperature environment, or prevent exfoliation of the energetic-beam absorber from the substrate due to thermal stress caused by operation in a low-temperature environment. When the deviation of the temperature of the selenium detector from the predetermined range is corrected by the above measures, the user can continuously use the energetic-beam detection apparatus without stopping the operation of the energetic-beam detection apparatus.
Preferably, the energetic-beam detection apparatus according to the fourth aspect of the present invention also has one or any possible combination of the aforementioned additional features (iii) to (v).
(5) According to the fifth aspect of the present invention, there is provided an energetic-beam detection apparatus comprising: a selenium detector which includes a substrate and an energetic-beam absorber being formed on the substrate and containing selenium; a temperature detecting unit which detects a temperature of the selenium detector; and a high-side temperature control unit which controls the temperature of the selenium detector so that the temperature is maintained equal to or below a predetermined upper limit.
Since the high-side temperature control unit controls the temperature of the selenium detector so that the temperature is maintained equal to or below a predetermined upper limit, the decrease in the practical lifetime of the selenium detector due to softening or development of bulk crystallization of selenium in a high-temperature environment can be automatically suppressed. Thus, even when the temperature of the selenium detector rises above the predetermined upper limit, users can continue to use the energetic-beam detection apparatus without being bothered with the temperature rise in the selenium detector. That is, the usability of the energetic-beam detection apparatus is improved.
Preferably, the energetic-beam detection apparatus according to the fifth aspect of the present invention also has one or any possible combination of the aforementioned additional features (v) and the following additional features (vi) to (xi).
(vi) The energetic-beam detection apparatus according to the fifth aspect of the present invention may further comprise a low-side temperature control unit which controls the temperature of the selenium detector so that the temperature is maintained equal to or above a predetermined lower limit. Since the low-side temperature control unit controls the temperature of the selenium detector so that the temperature is maintained equal to or above a predetermined lower limit, the exfoliation of the energetic-beam absorber from the substrate due to thermal stress caused by operation in a low-temperature environment can be automatically prevented. Thus, even when the temperature of the selenium detector drops below the predetermined lower limit, users can continue to use the energetic-beam detection apparatus without being bothered with the temperature drop in the selenium detector. That is, the usability of the energetic-beam detection apparatus is further improved. In addition, the energetic-beam detection apparatus can be used in a temperature range in which the detection sensitivity is high. Therefore, the detection sensitivity of the energetic-beam detection apparatus is improved.
(vii) The predetermined upper limit may be one of 40xc2x0 C., 45xc2x0 C., and 50xc2x0 C.
(viii) In the energetic-beam detection apparatus according to the fifth aspect of the present invention having the feature (vi), the predetermined lower limit may be one of 0xc2x0 C., 10xc2x0 C., and 30xc2x0 C.
(ix) In the energetic-beam detection apparatus according to the fifth aspect of the present invention having the feature (vi), the predetermined upper limit may be 40xc2x0 C., and the predetermined lower limit may be 30xc2x0 C. In this case, the energetic-beam detection apparatus can be used in a temperature range in which the detection sensitivity is high. Therefore, the detection sensitivity of the energetic-beam detection apparatus is improved.
(x) The high-side temperature control unit may comprise a cooling unit which cools the selenium detector. The cooling unit may be a mechanical cooling unit, such as a fan, which cools the selenium detector by blowing air. Alternatively, the cooling unit may be an electrical cooling unit such as a Peltier element. When the high-side temperature control unit comprises the cooling unit, the temperature of the selenium detector can be easily dropped to or below the predetermined upper limit.
(xi) In the energetic-beam detection apparatus according to the fifth aspect of the present invention having the feature (vi), the low-side temperature control unit may comprise a warming unit which warms the selenium detector. The warming unit may be a warm-air blower, heater, or the like. When the low-side temperature control unit comprises the warming unit, the temperature of the selenium detector can be easily raised to or above the predetermined lower limit.
In the first to fifth aspects of the present invention, the temperature of the selenium detector may be the temperature of the main body of the selenium detector or the temperature in the vicinity of the selenium detector.